Leto was the Titan mother of Artemis and Apollo in Greek mythology. Leto's father is usually given as Koios, but is unspecified by Homer. Leto's mother is Phoebe; her sister, Asterie. Sometimes Hekate is named a sister, too.
Why Was She on Delos?Leto had trouble finding a place to give birth to her children by Zeus. Persuading rocky Delos to allow Leto to give birth had been difficult. All the possible locations were reluctant to accept Leto because it would mean risking the wrath of Hera, the queen of the gods and wife of the father of Leto's children. Rocky Delos finally agreed, but on its terms. It required an oath on the River Styx that Leto's children would pay it honors.
The childbirth goddess Eileitheia had arrived days late -- having been delayed deliberately by Hera. By the time of her arrival, Leto was 9 days into her labor. With the childbirth goddess' help and also, perhaps, that of her daughter, especially according to the version in which Leto gave birth to Artemis on Ortygia, she gave birth to Apollo on Delos, while she leaned on Mt. Cynthos for support.
Pseudo-Apollodorus, Bibliotheca 1. 21:
"[Leto] finally reached Delos and gave birth to Artemis, who thereupon [as a baby] helped her deliver Apollon. Artemis became a practised huntress and remained a virgin."
In Greek art, Leto is shown with her children, mounting a chariot, or abducted by Tityos with or without an arrow in his head. Leto has a minor role in the Iliad.
Reference: Timothy Gantz Early Greek Myth.
Hesiod presents Leto as "gentle to men and gods".
Leto is involved in a few stories of Greek mythology. During the Trojan War, Leto helps heal Aeneas. When Niobe insulted Leto by claiming to be more blessed in children, Leto's children punished Niobe by killing her children. A giant called Tityus tried to rape Leto. For this crime, he was punished in Promethean-style in the Underworld.
Leto may be one of the characters depicted on the Corbridge Lanx.